powerpc/mm/radix: Add MMU_FTR_RADIX

We are going to add asm changes in the follow up patches. Add the
feature bit now so that we can get it all build.

mpe: When CONFIG_PPC_RADIX_MMU=n we omit MMU_FTR_RADIX from the
MMU_FTRS_POSSIBLE mask. This allows the compiler to work out that those
checks will always be false and so the code can be elided completely.

Note we do *not* define MMU_FTR_RADIX to 0 in the RADIX_MMU=n case,
because that doesn't work with the ASM_FTR patching. In particular an
IF_SET section will result in a mask and value of zero, which is always
true, meaning the section *won't* be patched, which is the opposite of
what we want.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Acked-by: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>